Mobile electronics devices require increasingly more efficient and rechargeable batteries to provide an independent power supply. Besides nickel-cadmium and nickel-metal hydride batteries, specially rechargeable lithium batteries, which have a substantially higher energy density compared to nickel batteries, are suitable for this purpose. The systems available on the market have a terminal voltage of ca. 3 V; this relatively high potential means that water-based electrolyte systems cannot be used in lithium batteries. Instead, non-aqueous, mostly organic electrolytes (i.e. solutions of a lithium salt in organic solvents such as carbonates, ethers or esters) are used in liquid systems.
In the currently predominating battery design—lithium ion batteries with liquid electrolytes—practically exclusively lithium hexafluorophosphate (LiPF6) is used as conducting salt. This salt satisfies the necessary preconditions for use in high-energy cells, i.e. it is readily soluble in aprotic solvents, leads to electrolytes with high conductivities, and has a high level of electrochemical stability. Oxidative decomposition occurs only at potential values greater than ca. 4.5 V. LiPF6 has serious disadvantages however, which can mainly be attributed to a lack of thermal stability (it decomposes above ca. 130° C.). Also, on contact with moisture corrosive and poisonous hydrogen fluoride is released, which on the one hand complicates handling and on the other hand attacks and damages other battery components, for example the cathode.
Given this background, intensive efforts have been made to develop alternative conducting salts, and in particular lithium salts with perfluorinated organic radicals have been tested. These salts include lithium trifluoromethane sulfonate (“Li-triflate”), lithium imides(lithium-bis(perfluoroalkylsulfonyl)imides) as well as lithium methides(lithium-tris(perfluoroalkylsulfonyl)methides). All these salts require relatively complicated production processes and are therefore relatively expensive, and have other disadvantages such as corrosiveness with respect to aluminium or poor conductivity.
Lithium organoborates have been investigated as a further class of compounds for use as conducting salt in rechargeable lithium batteries. On account of their low oxidation stability and safety considerations in the handling of triorganoboranes, their use for commercial systems is excluded however.
The lithium complex salts of the type ABL2 (where A denotes lithium or a quaternary ammonium ion, B denotes boron and L denotes a bidentate ligand that is bound via two oxygen atoms to the boron atom) proposed in EP 698301 for use in galvanic cells represent a considerable step forward. The proposed salts, whose ligands contain at least one aromatic radical, however only have a sufficient electrochemical stability if the aromatic radical is substituted by electron-attracting radicals, typically fluorine, or if it contains at least one nitrogen atom in the ring. Such chelate compounds are not commercially available and can be produced only at high cost. The proposed products could therefore not penetrate the market.
Very similar boron compounds are proposed in EP 907217 as constituents in organic electrolytic cells. Compounds of the general formula LiBXX′ are proposed as boron-containing conducting salt, where the ligands X and X′ may be identical or different and each ligand contains an electron-attracting, oxygen-containing group that bonds to the boron atom. The listed compounds (lithium-boron disalicylate and a special imide salt) exhibit the disadvantages already mentioned above however.
The compound lithium-bis(oxalato)borate (LOB) described for the first time in DE 19829030 is the first boron-centred complex salt described for use as an electrolyte that employs a dicarboxylic acid (in this case oxalic acid) as chelate component. The compound is simple to produce, non-toxic and electrochemically stable up to about 4.5 V, which permits its use in lithium ion batteries. A disadvantage however is that it can hardly be used in new battery systems with cell voltages of >3 V. For such electrochemical storage units salts with stabilities of ≧ca. 5 V are required. A further disadvantage is that lithium-bis(oxalato)borate does not allow any possible structural variations without destroying the basic chemical framework.
In EP 1035612 additives inter alia of the formulaLi+B−(OR1)m(OR2)p are mentioned,and m and p=0, 1, 2, 3 or 4, where m+p=4, and                R1 and R2 are identical or different and are optionally directly bonded to one another by a single or double bond, in each case individually or jointly denote an aromatic or aliphatic carboxylic acid or sulfonic acid, or in each case individually or jointly denote an aromatic ring from the group comprising phenyl, naphthyl, anthracenyl or phenanthrenyl, which may be unsubstituted or singly to quadruply substituted by A or Hal, or in each case individually or jointly denote a heterocyclic aromatic ring from the group comprising pyridyl, pyrazyl or bipyridyl, which may be singly to triply unsubstituted or substituted by A or Hal, or in each case individually or jointly denote an aromatic hydroxy acid from the group comprising aromatic hydroxycarboxylic acids or aromatic hydroxysulfonic acids, which may be singly to quadruply unsubstituted or substituted by A or Hal, and                    Hal=F, Cl or Br, and            A=alkyl radical with 1 to 6 C atoms, which may be singly to triply halogenated.                        
As particularly preferred additives there may be mentioned lithium-bis[1,2-benzenediolato(2-)O,O′]borate(1-), lithium-bis[3-fluoro-1,2-benzenediolato(2-)O,O′]borate(1-), lithium-bis[2,3-naphthalenediolato(2-)O,O′]borate(1-), lithium-bis[2,2′-biphenyldiolato(2-)O,O′]borate(1-), lithium-bis[salicylato(2-)O,O′]borate(1-), lithium-bis[2-olatobenzenesulfonato(2-)O,O′]borate(1-), lithium-bis[5-fluoro-2-olatobenzenesulfonato(2-)O,O′]borate(1-), lithium phenolate and lithium-2,2-biphenolate. These are all symmetrical lithium chelatoborates of the type Li[BL2].
Lithium-bis(malonato)borate, which is said to have an electrochemical window up to 5 V, has been described by C. Angell as being an electrochemically particularly stable simple lithium-(chelato)borate compound. The compound in question has the disadvantage however that it is practically insoluble in the usual battery solvents (e.g. only 0.08 molar in propylene carbonate), which means that it can be dissolved and characterised only in DMSO and similar prohibitive solvents for batteries (Wu Xu and C. Austen Angell, Electrochem. Solid-State Lett. 4, E1-E4, 2001).
Chelatoborates may furthermore be present in protonated form (i.e. H[BL2]) where L is a bidentate ligand that is bound to the boron atom via two oxygen atoms. Such compounds have an extremely high acidic strength and may therefore be used as so-called super acids in organic synthesis as catalysts for cyclisations, aminations, etc. For example, hydrogen-bis(oxalato)borate has been proposed as a catalyst for the production of tocopherol (U.S. Pat. No. 5,886,196). The disadvantage of this catalyst however is the relatively poor hydrolytic stability.
The object of the present invention is to obviate the disadvantages of the prior art and in particular to provide substances for conducting salts that can be produced relatively simply and inexpensively from commercially available raw materials, that have a sufficient oxidation stability of at least 4.5 V, and that are readily soluble in conventionally used “battery solvents”. Furthermore the substances should be relatively resistant to decomposition by water or alcohols.
This object is achieved by “mixed” boron chelate complexes of the general formula
where                X is either —C(R1R2)— or —C(R1R2)—C(═O)—, in which        R1, R2 independently of one another denote H, alkyl (with 1 to 5 C atoms), aryl, silyl or a polymer, and one of the alkyl radicals R1 or R2 may be bonded to a further chelatoborate radical,                    or X denotes 1,2-aryl with up to two            substituents S in the positions 3 to 6                        
in which S1, S2 independently of one another denote alkyl (with 1 to 5 C atoms), fluorine or a polymer,                as well as M+ denotes Li+, Na+, K+, Rb+, Cs+ or [(R3R4R5R6)N]+ or H+, where R3, R4, R5, R6 independently of one another denote H or alkyl with preferably 1 to 4 C atoms.        
It has surprisingly been found that the aforelisted borates having two different ligands, one of which is the oxalato radical, have significantly better solubilities than the symmetrical comparison compounds.
The following compounds are preferred: hydrogen-(malonato,oxalato)borate (HMOB), hydrogen-(glycolato,oxalato)borate (HGOB), hydrogen-(lactato,oxalato)borate (HLOB), hydrogen-(oxalato, salicylato)borate (HOSB) and bis-[hydrogen-(oxalato,tartrato)borate] (BHOTB), as well as the lithium, caesium and tetraalkylammonium salts of the aforementioned acids.
The solubility of the mixed borates, which is better compared with that of the symmetrical comparison compounds, is demonstrated in Table 1 by the example of the lithium compounds:
TABLE 1Solubilities of various lithium borate complexsalts (in mole/kg) at room temperatureLOBLLBLMBLOMBLLOBLOSBTHF1.90<0.010.090.171.591.21PC0.88<0.010.020.170.171.50γ-BL1.550.020.130.620.960.981,2-DME1.30<0.010.0030.200.930.43Acetone1.82<0.010.030.431.380.51LOB = Lithium-bis(oxalato)borateLLB = Lithium-bis(lactato)borateLMB = Lithium-bis(malonato)borateLOMB = Lithium-(malonato,oxalato)borateLLOB = Lithium-(lactato,oxalato)borateLOSB = Lithium-(oxalato,salicylato)borateTHF = TetrahydrofuranPC = Propylene carbonateγ-BL = γ-Butyrolactone1,2-DME = 1,2-dimethoxyethane
It is clear that the compound LOB not according to the invention has in most cases the best solubility. Surprising however compared to the other symmetrical compounds (LMB and LLB) is the substantially improved solubility of the mixed chelatoborates. In the solvent propylene carbonate the mixed LOSB is even substantially more soluble than LOB.
Table 2 shows the hydrolysis susceptibility of various chelatoborates.
TABLE 2Degree of hydrolysis of 5% solutions in water afterstirring for 2 hours at room temperatureLOBLMBLMOBDegree of hydrolysis (%)>50515
The metal salts with mixed boron chelate anions according to the invention can dissolve in relatively high concentrations in the typical aprotic solvents such as carbonates, lactones and ethers used for high-performance batteries. Table 3 gives the measured conductivities at room temperature:
TABLE 3Conductivities of non-aqueous electrolytes withmixed chelatoborate salts in γ-BL, 1,2-DME andTHF at room temperatureγ-BL1,2-DMETHFConcn.1)Cond.2)Concn.1)Cond.2)Concn.1)Cond.2)LLOB0.962.610.936.521.592.91LSOB0.434.171.213.17LMOB0.543.590.170.41LMB0.131.65 0.0090.01LLB0.020.01insoluble0insoluble0LOB1.046.961)in mmole/g2)in mS/cm
It can be seen from Table 3 that solutions of the mixed borate salts have conductivities of >2 mS/cm necessary for the operation of lithium batteries. In contrast to this, the solutions not according to the invention of the symmetrically substituted salts LMB and LLB have significantly lower or practically zero conductivities.
The conductivities may be optimised corresponding to the prior art by for example combining at least one solvent having a high dielectric constant (for example ethylene carbonate, propylene carbonate) with at least one viscosity-reducing agent (for example dimethyl carbonate, butylacetate, 1,2-dimethoxyethane, 2-methyltetrahydro-furan).
The salts with mixed chelatoborate anions furthermore exhibit the desired high degree of electrochemical stability. For example the compound lithium-(lactato,oxalato)borate according to the invention has a so-called “electrochemical window” of ca. 5 V, i.e. it is stable in the range between 0 and ca. 5 V (Li/Li+=0), as shown in FIG. 1.
The boron chelate complexes described above can be fixed to polymer compounds by known techniques. Thus, it is possible to remove the acidic hydrogen atoms in the α-position to the carbonyl groups by means of suitable bases and to add the carbanionic species obtained in this way to functionalised (e.g. halogenated) polymers.
The boron chelate complexes according to the invention can be produced by reacting boric acid or boron oxide with oxalic acid and the other chelate-forming agent, optionally in the presence of an oxidic metal source (e.g. Li2CO3, NaOH, K2O) or an ammonium salt, for example according to the following equations:H3BO3+C2O4H2+L2→H[B(C2O4)L2]+3H2Oor0.5B2O3+C2O4H2.2H2O+L2+LiOH.H2O→Li[B(C2O4)L2]+5.5H2Owhere L2 denotes dicarboxylic acid (not oxalic acid), hydroxycarboxylic acid or salicylic acid (which may be at most doubly substituted).
Preferably stoichiometric amounts of the starting substances are used, i.e. the molar ratio boron/oxalic acid/chelate-forming agent L2/optionally oxidic metal source or ammonium salt is about 1:1:1:1. Small deviations from the theoretical stoichiometry (e.g. 10% above or below) are possible without having a marked effect on the chelatoborate end product. Thus, if one of the ligands is present in excess, the corresponding symmetrical end product will occur to a greater extent in the reaction mixture. Thus for example if >1 mole of oxalic acid is used per equivalent of boron-containing raw material, then bis-(oxalato)borate will be formed in significant amounts. If the reaction is carried out in the presence of an oxidic lithium raw material LOB is formed, which in combination with the mixed chelatoborates according to the invention does not interfere in applications as a battery electrolyte. If on the other hand an acid L2 whose symmetrical chelato compound is sparingly soluble is employed in excess, then the byproduct M[B(L2)2] can easily be separated by a simple dissolution/filtration step. It is important to use ca. 2 moles of chelate-forming agent per equivalent of boron component. If the chelate-forming agent is not used in a stoichiometric amount, unreacted boron component or undesirable 1:1-adduct (HO—B(C2O4) or HO-BL2) will remain. If more than 2 moles of chelate-forming agent are used, then unreacted chelate-forming agent will remain, which has to be separated in a complicated procedure.
The reaction according to the above chemical equations is preferably carried out by suspending the raw material components in a medium suitable for the azeotropic removal of water (e.g. toluene, xylene, methylcyclohexane, perfluorinated hydrocarbons with more than 6 C atoms) and removing the water azeotropically in a known way.
It is also possible to carry out the synthesis in aqueous solution. In this case the components are added to water in an arbitrary sequence and are concentrated by evaporation while stirring, preferably under reduced pressure. After removing most of the water a solid reaction product forms which, depending on the specific product properties, is finally dried at temperatures between 100° and 180° C. and under reduced pressure (e.g. 10 mbar). Apart from water, alcohols and other polar organic solvents are also suitable as reaction media.
Finally, the product can also be produced without adding any solvent, i.e. the commercially available raw materials are mixed and then heated by supplying heat and dehydrated preferably under reduced pressure.
The acids H[BC2O4L2] produced in this way are used in organic synthesis as super acid catalysts, e.g. for condensations, hydroaminations or debenzylations. Lithium salts of the mixed chelatoborates are used as electrolytes in galvanic cells, preferably lithium batteries. The ammonium and caesium salts may be used in electrolytic double-layer capacitors.